The invention concerns a method for sensing the faulty positioning of an optically evaluatable test strip in a measuring device for measuring a substance in a fluid, especially for blood sugar determination, wherein the fluid to be investigated is applied to the test field of a test strip and the change in the reflection or transmission ability of the test field caused thereby is captured and evaluated.
In a blood sugar measurement carried out by a patient itself the patient applies a drop of blood onto the test field of a test strip which test field is then optically measured. Thereby, a detector for example captures the color change of the test field, which is caused by the dropping of the blood onto the test field. A correct value is then obtained only if previously a correct empty value measurement has been carried out on the unused test field. If, for example, the test strip is not correctly inserted into the measuring device, the measuring optic system then does not sense the unused test field but instead senses the carrier foil of the test strip which differs from the test field by having a different optical reflection or transmission characteristic. In this case the empty measurement is a mistaken one which necessarily leads to incorrect measurement results.
A method is known from WO 96 07 907 for detecting the faulty positioning of an optically evaluatable test strip in order to enable one to determine whether the test strip is correctly inserted into the measuring device, that is without the test strip being oriented upside down. In this case the test strip in the area between its input end and the test field has a so called orientation indication, whose reflection ability differs from the reflection ability of the remaining portion of the upper surface of the test strip carrier material. Upon insertion of the test strip this orientation indicator zone, for example a black bar on a white carrier material, is captured by the measuring optic system, as a result of which the device presumes that the test strip has been correctly inserted into the measuring device. If the measuring optic system on the other hand does not sense this orientation indicator zone the measuring device presumes that the strip has been incorrectly inserted into the device. The patient is informed of this by an indication on the indicator mechanism of the measuring device to check and change the position of the measuring strip. This device cannot however determine whether the test strip has been correctly so fully inserted into the measuring device that the measuring field can in its entirety be captured by the measuring optic system.
The invention has as its object the provision of a method which avoids the above mentioned difficulty and allows a determination to be made of whether the test strip is correctly positioned relative to the measuring optic systems so that only the test field, and not in a faulty way, a portion of the upper surface of the carrier of the test strip is captured by the measuring optic system.
The above mentioned object is solved by a method of the above-mentioned kind wherein the measuring field for measuring the test field of the test strip is divided into at least two measuring areas with the different measuring areas being arranged behind one another in the insertion direction of the test strip and with the separate measuring areas being separately sensed, and wherein the test strip has zones of different reflection or transmission abilities which zones lie behind one another and in front of the test field in respect to the insertion direction of the test strip, and wherein the difference between the measured values obtained for the two measuring areas is formed and compared with a pregiven threshold value, and a faulty positioning indicating signal being produced when the difference between the measured values exceeds the threshold value.
If the test strip is correctly positioned in the measuring device both measuring areas of the device lie within the test field. The measuring optic systems associated with the measuring areas capture only areas of the test field which in normal cases, in the empty value measurement as well as in the actual measurement, deliver at least nearly equal reflection or transmission values. On the other hand, if the test strip is not correctly positioned relative to the measuring optic systems, for example not inserted deeply enough into the device, the two measuring areas fall either onto zones of different reflection or transmission values or one of the measuring areas falls into the measuring field while the other measuring area comes into registration with one of the mentioned zones. In any event, the two measuring optic systems associated with the measuring areas in this case deliver such different values that the mentioned threshold value is exceeded, and accordingly a corresponding indication is produced by means of which the user is, for example, instructed to check the position of the test strip in the measuring device and, if needed, to correct the position.
The invention further concerns a test strip, for carrying out the above described method, with a carrier and a test field for receiving the investigated liquid, with the carrier in accordance with the invention at least in the area which, with reference to the test strip insertion direction, lies in front of the test field having zones of different reflection or transmission abilities, the dimensions of which zones in the insertion direction at least nearly correspond to the measurements of the measuring areas. Preferably, the zones of different reflection or transmission abilities are formed by strips of different color, which for example can be printed onto the carrier material of the test strip.
As the measuring apparatus, a measuring device can be used such as that described in German Utility Model Registration No. 2 962 037 2 of the applicant with the restriction that the measuring optic systems associated with the two measuring areas be arranged behind one another in the test strip insertion direction.
The following description explains the invention in connection with the accompanying drawing by way of an exemplary embodiment.